The present invention relates to fiber optics, and more particularly to high power fiber amplifiers and lasers and devices for optically pumping these in order to create a population inversion within an active medium of such amplifiers and/or lasers.
Double cladding fiber structures, as described in U.S. Pat. No. 4,815,079 to Snitzer, et al., have been demonstrated to be an effective approach for implementing high power fiber lasers and amplifiers. See L. Goldberg et al., xe2x80x9cHighly efficient 4-W Yb-doped fiber amplifier pumped by a broad stripe laser diode,xe2x80x9d Optics Letters, v. 15, pp. 673-675, 1999. In a double cladding fiber a single mode doped core is surrounded by a large, multimode inner cladding, which is in turn surrounded by an outer cladding. To achieve a large numerical aperture (NA) in the inner cladding, an outer cladding with a low refractive index is chosen, typically resulting in an inner cladding NA of 0.25-0.6. Typical inner cladding diameters are 100-200 xcexcm to allow efficient coupling of emission from 100-200 xcexcm wide, high power, broad stripe laser diodes. Compared with conventional single mode fiber amplifiers, the double cladding fiber offers several important advantages. First, high power non-diffraction limited laser diodes, such as broad area diodes can be used to pump the fiber. Such diodes can generate 2-4 W from a 100 xcexcm-200 xcexcm aperture, compared with 100-200 MW available from single mode pigtailed diodes used to pump conventional fiber amplifiers. Second, since in double cladding fibers the pump light is coupled into a large, multimode inner cladding, the diode-to-fiber alignment tolerance is at least one order of magnitude larger than that associated with single mode diode pigtailing.
An effective method for coupling the emission of a broad area pump diode into the inner cladding of a double cladding fiber is through a v-groove fabricated into the fiber, as shown in U.S. Pat. No. 5,854,865, entitled xe2x80x9cMethod and apparatus for side-pumping of optical fiberxe2x80x9d to Goldberg. The method makes it possible to couple the pump light directly into the fiber with high efficiency. See L. Goldberg et al., xe2x80x9cHighly efficient 4-W Yb-doped fiber amplifier pumped by a broad stripe laser diode,xe2x80x9d Optics Letters, v. 15, pp. 673-675, 1999. The method can be implemented with or without using any lenses between the diode and the fiber. See L. Goldberg et al. xe2x80x9cDouble cladding fiber amplifiers with lens-less side-pumping,xe2x80x9d paper CFC1, CLEO, 2000. FIG. 1a shows such a lens-less coupling arrangement, where the fiber is attached to a glass substrate, typically 200 xcexcm thick, to provide mechanical support for the fiber. Light incident on the v-groove facets undergoes total internal reflection at the glass-to-air interface and is coupled in both directions into the fiber. The broad stripe diode is oriented with its junction plane perpendicular to the longitudinal fiber axis so that light is incident on the right and left v-groove facets at angles less than 45 degrees relative to the facet surface. This is below the critical angle of 47 degrees required for total internal reflection at a surface of Silica with a refractive index of 1.46. Because the beam diverges rapidly as it propagates through the diode-to-substrate air gap, the glass substrate, and the fiber, it overfills the v-groove, resulting in a decrease of the coupling efficiency. For a typical substrate thickness of 200 xcexcm and a fiber diameter of 130 xcexcm, a coupling efficiency of only 40% has been achieved in a lens-less coupling arrangement. See L. Goldberg et al. xe2x80x9cDouble cladding fiber amplifiers with lens-less side-pumping,xe2x80x9d paper CFC1, CLEO, 2000. This compares unfavorably with a coupling efficiency of approximately 90% demonstrated when a microlens was used between the diode and the fiber. See L. Goldberg et al., xe2x80x9cHighly efficient 4-W Yb-doped fiber amplifier pumped by a broad stripe laser diode,xe2x80x9d Optics Letters, v. 15, pp. 673-675, 1999. Using improvements described in this invention, the coupling efficiency for the lens-less arrangement can be increased substantially and the method can be used to construct a hermetically sealed pump module.
The present invention comprises an apparatus of and a method for constructing fiber amplifiers and lasers with hermetically sealed laser diode pump modules and passive mechanical alignment of all critical components. An embodiment of the present invention uses a double cladding fiber and a v-groove side-pumping technique that directly couples pump light into the fiber without the use of lenses. The method and apparatus of an embodiment of the present invention hermetically seals the pump diode by having the transparent substrate on which the fiber is mounted also serve as the package window.
Under certain conditions, as described herein, the package window serves to transmit pump light from the diode to the fiber, while greatly reducing the diode beam divergence. This allows efficient coupling of the diode light into the fiber without using lenses, normally required to capture and focus the highly divergent light generated by a laser diode. The large alignment tolerances associated with coupling the pump light into a large multimode fiber cladding are much greater than those associated with single mode diode pigtailing. This large alignment tolerance, in conjunction with the simplified method of coupling the pump light into the double cladding fiber, makes it possible to fabricate the fiber pigtailed pump module using passive mechanical alignment. These improvements make it possible to avoid resorting to active alignment methods associated with fabrication of single mode pump modules. The pump packaging method and apparatus described herein make provisions for achieving passive mechanical alignment of the fiber and the v-groove along all three orthogonal axis, xyz. The amplifier and pump module apparatus and construction method described herein can substantially lower the cost of fiber amplifiers. The lowered cost is particularly true for those fiber amplifiers with moderate and high power output, which with conventional pump couplings would require multiple and costly pump diode modules. Moreover, the present invention greatly increases the coupling efficiency over that previously achieved in lens-less couplings.
One of the features of an embodiment of the apparatus and method of the present invention is hermetic packaging of double cladding pump modules. By having a non-stripped section of the double cladding fiber outside the hermetically sealed pump module, the difficult problem of achieving a hermetic seal to a polymer fiber jacket or outer cladding is avoided. The packaging approach used with conventional single mode fiber pigtails, where hermetic seal is achieved by using epoxy or soldering the bare (polymer jacket removed) fiber inside a ferrule, generally cannot be used with double cladding fibers. This is because there are presently no epoxies that provide a satisfactory hermetic seal and have a refractive index substantially below that of the Silica glass (n=1.46) of the inner cladding. As is described herein, the low refractive index is usually required to maintain the high numerical aperture of the inner cladding, and to prevent leakage of the pump light from the inner cladding into the epoxy. In addition, in the disclosed design the low refractive index polymer adhesive, required to bond the fiber to a transparent substrate, is placed outside of the diode package. Such polymers are kept outside the hermetically sealed package to avoid outgassing.
Another feature of an embodiment of the apparatus and method of the present invention is direct and efficient coupling of the pump diode emission into the double cladding fiber without using lenses. By using a high refractive index window in the hermetic package, the diode beam divergence is reduced to allow coupling efficiencies above 80%. At the same time, the method allows sufficient window thickness to provide mechanical strength, and sufficiently large diode-to-window spacing to make passive alignment possible.
Another feature of an embodiment of the apparatus and method of the present invention is that the disclosed coupling and packaging provides sufficiently large alignment tolerances to allow passive mechanical alignment of all critical components. This significantly simplifies the pump module and fiber amplifier assembly process and lowers the overall manufacturing cost. The passive alignment method also makes the implementation of an automatic assembly process much easier than is possible when precise active alignment is required, as in the case of single mode fiber pigtailing. Another benefit of the large alignment tolerance is that the alignment and diode-to-fiber coupling are insensitive to thermal and mechanical perturbations.
Another feature of an embodiment of the apparatus and method of the present invention is that the disclosed package design provides a mechanical means for achieving passive alignment of the diode and the fiber.